Answer:
order fulfilment and logistics
Explanation:
Based on the information provided within the question it can be said that the problem that these online retailers are facing are order fulfilment and logistics. This is because (as described in the question) processing huge volumes of small orders takes up too much time in the logistics side of the business such as finding the product in the warehouses and making sure the correct labels are applied, and this causes many orders to not be fulfilled because of it.